1. Field of the Invention
The invention relates to an electro-optical system for projecting very high brightness, full color images from television or other program sources through a projection lens onto a display screen.
2. Prior Art
Full color image display systems for projection onto a large screen suitable for group viewing of images from television or similar program sources are desirable in applications such as educational, governmental or business conference viewing, cafe or theater entertainment and the like. The efforts to develop such systems have not heretofore overcome the problem of simultaneously achieving the full brightness, the clarity and the resolution to which theater patrons have become accustomed in viewing motion picture film projection. Presently available theater television projection systems have not been able to achieve this standard in spite of many attempts.
Some of these attempts have involved the use of Schlieren optical systems which provide a dark field projector in the sense that no light reaches the screen in the absence of an image signal. This is achieved by providing a diffraction grating or mesh grid both before and after a modulating surface in an optical light path. In the absence of signal, light is blocked by the mesh grid and is transmitted only when video signal changes an optical property of the modulating surface, such as its local curvature, in order to cause a change in the refraction of light thus permitting it to pass through the second grid. Such systems are inherently limited in the amount of usable light and have limited resolution. One such system is, for example, described in U.S. Pat. No. 3,835,346 to F. Mast. In addition to the above noted problems this system is inherently directed to a black and white or monochromatic imaging system. It does have the merit of recognizing the advantage of using an independent projection light source to overcome the limited light intensity available from systems which attempt a direct projection of a cathode ray tube generated image.
One earlier version of such a Schlieren System is described by E. Labin in an article entitled "The Eidophor Method of Theater Television" at page 393 of the April 1950 issue of the Journal of the SMPTE. Another early version of such a system was disclosed by E. Baumann in U.S. Pat. No. 2,892,380 (June 30, 1959). Like Mast, Baumann realizes the need for using an independent projection light source. Baumann modulates this projection light by means which he refers to as a multiple layer control means including a combination of a photoconductive layer with a layer of a substance with electric Kerr effect such that the index of refraction of the latter substance is varied by an electric field resulting from a low level light signal impinging on the photoconductor. The combination of a Schlieren optical system with what appears to be intended to be a liquid crystal material operated in the dynamic scattering mode, however, can not give the image contrast, speed of response, or effective light utilization required for efficient projection system as can be seen from the low effectiveness of present day commercial systems.
Also, Baumann, like Mast, is directed to a monochromatic system. Representative approaches to a color projection system are shown in U.S. Pat. Nos. 2,958,258 to Kelly and 3,893,758 to Hunzinger. Insofar as the Kelly patent relates to a full color system utilizing three primary colors, it is inherently a serial mode subtractive system rather than a parallel mode additive system. This feature alone inherently detracts from its possible efficiency level. Kelly states beginning at line 65 of column 1 that the principle object of his invention is to provide a projection system wherein an independent light source can be arranged off of the axis of a modulating object to avoid the skewing problems he has previously discussed and wherein light from this source can be conducted into and out of a transparent modulator one side of which is inaccessible to this light. To do this he provides a polarization selective light dividing interference coating of the type described in U.S. Pat. Nos. 2,403,731 to MacNeille or 2,449,287 to Flood. The polarized light output from this device is supplied to one or more cathode ray tubes, each with a scotophor layer which subtracts a predetermined portion of the color spectrum. Reference is made to the description of FIG. 3 beginning at line 30 of column 4 of Kelly. Thus, in all of his three color systems, Kelly takes light from a lamp L and transmits it in series through a plurality of polarizing beam splitters, first to a scotophor tube which subtracts one color from the white beam, then to a second scotophor which subtracts a second color modulation pattern from the beam, and then the same light is transmitted to a third scotophor which subtracts the third color from the beam before the final beam is projected onto the screen. The same light goes sequentially to all three scotophor tubes for subtractive modulation thus posing serious problems of image registration and efficiency of light use. In particular, such absorption devices make it difficult to achieve necessary brightness for large scale use without overheating the system. Furthermore, in association with each of his scotophor tubes Kelly places a quarter waveplate and sometimes an additional filter. Each of these components introduces additional possibilities of imperfection in execution of the system.
The Hunzinger U.S. No. 3,893,758 overcomes some of the problems in Kelly by substituting a Wollaston polarizing prism for the MacNeille prism used in Kelly. This change precludes the use of orthogonal optic axes since the Wallaston prism is not polarization selective at a 45.degree. or 90.degree. incidence angle. Furthermore, whereas the MacNeille prism can be of any reasonable area, such as several square inches, since it consists of a deposited multilayer interference filter at the interface of two prisms, the Wollaston polarizing prism is actually composed of the interface between two naturally occurring single crystals which normally occur with an average diameter of approximately 1 centimeter. The result of this limitation is that light from the projection light source must be focused onto the polarizing prism by a curved mirror 123 and is then passed through lenses such as 34 or 35 before being transmitted to the modulating cathode ray tubes 31, 32 and 33. The necessity of focusing light on the polarizer entails the necessity of recollimating it by lenses before it reaches the modulation elements which in turn introduces all of the problems of optical aberration inherent in any non-collimated optical system. Furthermore, the oblique angle critically imposed by the Wollaston prism makes the mechanical design in the projector system difficult to lay out and also imposes stringent alignment requirements. For his modulation device Hunzinger turns to cathode ray tubes of the optical relay type wherein the target is in the form of a plate of KD.sub.2 PO.sub.4 material (KDP) exhibiting the electro-optical Pockels-effect such that the video image written on the back of the face plate of the tube by the electron beam produces a change in the birefringence of the crystalline material of the plate which modulates polarization of the beam. The quantitative value of the birefringence of such plates, however, is much lower than can be obtained from presently available liquid crystal materials with the results that the KDP crystal plate must be considerably thicker than a layer of liquid crystal in order to achieve the necessary polarization modulation. This thickness in turn requires that high voltages be applied to the plate. Furthermore the plate requires an elaborate cooling system (not shown in Hunzinger's drawing) since it must be operated at -60.degree. C. Such materials are only birefringent when they are held below the Curie point for the material. Such a cooling system requirement renders the system unsuitable for use in any but the most elaborately provided permanent insulations.
Image brightness amplifiers using liquid crystal materials are disclosed, for example, in U.S. Pat. Nos. 3,803,408 to Assouline et al., 3,732,429 to Braunstein et al., and 3,824,002 to Beard. Assouline shows a D.C. light valve whereas Beard shows an alternating current driven light valve. All three use liquid crystal materials operating in the dynamic scattering mode which cannot give the image contrast and speed of response required for the present system application.
An AC field effect mode of operating a birefringent liquid crystal material was reported in an article entitled "Electric-Field-Induced Orientational Deformation of Nematic Liquid Crystals: Tunable Birefringence" in "Applied Physics Letters" volume 20, No. 5, Mar. 1, 1972 beginning at page 199 by F. J. Kahn. Kahn therein described a laboratory display device for exhibiting spatially uniform birefringences tunable over the range of 0.0 to 0.15 for applied voltages less than 20 volts rms and with a sharp threshold below 4 volts rms for thin layers of nematic liquid crystals with negative dielectric anisotropy. The device consisted of a thin film of nematic material sandwiched between two electrodes at least one of which was transparent. An electric field applied along the normal to the sample layer is used to tune the birefringence, the variation of which was observed by sandwiching the package between crossed or parallel polarizers. A somewhat similar liquid crystal display device also incorporating matrix type electrodes for defining numerals is described in U.S. Pat. No. 3,900,248 to Nagasaki.
An image brightness amplifier including a field effect mode birefringent liquid crystal material light valve to obtain spatially modulated birefringence effects in a light valve and single channel projection system is disclosed and claimed in copending U.S. Patent application Ser. No. 538,381, filed Jan. 6, 1975 by W. P. Bleha et al., now abandonded, and assigned to the same assignee as the present application. That application is directed to an AC field effort mode liquid crystal light valve for spatially modulating the polarization of projection light in response to a low level light image as from a cathode ray tube which is of the type suitable for use in the system described herein. That application, however, is directed to use of such a light valve only in system having a single optical channel suitable for monochromatic black and white or color symbology projection. The present application is directed to a full color projection system which may use three such light valves.